Role of the inter-fascicular matrix in age related deterioration of tendon mechanical function

Tendon injury is very common, and the risk of injury increases with age. Some tendons are more likely to be injured than others, e.g. the human Achilles is highly prone to injury. This is partly due to its function. While most tendons are involved in limb placement for movement, some, including the Achilles have another function; they act as a spring, stretching to store energy then recoiling to return it, decreasing the body's energy consumption. To achieve this, they have a specialist structure. All tendons are composed of collagen molecules, grouped together to form larger subunits, the largest of which is the fascicle. Fascicles are bound together by inter-fascicular matrix (IFM), but we have evidence that this matrix has a unique and important mechanism in energy storing tendons and is responsible for allowing these tendons to act as springs. This has exciting implications; some individuals are able to use their tendons heavily without injury occurring whilst others are very prone to injury. We believe this may occur as a result of variations in the IFM, reducing the tendon capacity to extend and recoil. If we can quantify IFM structure and function and relate this to tendon properties, we may be able to predict the likelihood of injury in specific tendons, and develop methods to increase resistance to tendon injury and prevent disability.

Racehorses also suffer from tendon injuries. These are one of the most common reasons horses used for athletic purposes are retired and is a cause for considerable economic loss. The energy storing superficial digital flexor tendon (SDFT) in horses is functionally equivalent to the human Achilles and is injured the most often. The characteristics of tendon injury are similar between horses and humans, so we can use the horse as a model to better understand the role of the IFM in human tendon injury. In horse tendon, our previous work shows that the SDFT has a less stiff IFM than the non-energy storing common digital extensor tendon (CDET), which enables the SDFT to extend further before failure. We also have evidence that SDFT fascicles have a better recoil mechanism, indicating that lower IFM stiffness is critical for increasing damage resistance and protecting tendons from injury. Further, we have evidence that the IFM becomes stiffer with age, which may explain why tendon injuries increase in older people and animals. We intend to compare the structure and mechanics of the IFM in different tendons and different aged horses to see if we can identify the specific molecules that are key in influencing IFM mechanical properties.

We will determine how the composition of the IFM influences its mechanical properties in both the SDFT and CDET. Firstly, we will look at the different proteins in the IFM, and establish how they are organised. We will then measure IFM stiffness and determine how this is influenced by different IFM components. Once we have identified structural differences in the IFM between tendon types, we will focus on the damage resistant SDFT for the rest of the project. We will perform several mechanical tests on the SDFT, which will allow us to establish which IFM components control the ability of the tendon to extend and recoil, and provide damage resistance. We will also identify any age related changes in IFM composition and structure, and link these with ageing changes in mechanical properties.

In this work, we will focus on horse tendon. However, our findings will be applicable to humans and other species. The outcomes of this work will be the identification of components that are important in preventing tendon injury. The resulting data will allow us to understand the role of the IFM in normal tendon function, and how this changes with age and injury. This work will help researchers to develop treatments targeted at the IFM, and even improve training plans to promote appropriate IFM development.

We have preliminary evidence that the interfascicular matrix (IFM) is a key determinant of tendon failure properties and fatigue resistance. We hypothesise that injury prone energy storing tendons have an extensible IFM with minimal hysteresis, leading to a structure with improved fatigue resistance. Poor optimisation of the IFM or structural changes with age may predispose individuals to tendinopathy.

This project aims to investigate the role of the IFM in modulating fatigue resistance. Using an equine model, we will compare IFM composition and mechanics between a high strain energy storing and a low strain positional tendon, correlating these data with the mechanical characteristics of the tendons. Looking specifically at the energy storing tendon, we will then establish how the IFM changes with ageing and how this influences fatigue resistance. Finally, we will use knockout models and enzymatic techniques to manipulate IFM composition, and establish how this influences our findings.

The equine model proves an ideal energy storing tendon, with similar properties to the human Achilles. It is also large enough to be used for a series of experiments, enabling paired statistical analyses of the influence of IFM organisation and age on tendon function. IFM composition will be assessed using histology, immunohistochemistry, mass spectroscopy and qPCR. IFM, fascicle and tendon mechanical properties will be determined using quasi-static and cyclic fatigue tests to failure.

Data will provide greater understanding of how injury and ageing influence tendon mechanics via the IFM and may provide insights into methods of limiting injury risk. This will inform treatment practices, and facilitate the design of targeted drugs or treatments for the IFM. It may also be possible to develop training protocols to encourage appropriate IFM development, with the long term goals of decreasing the incidence of tendon injury and improving recovery rate post-injury.

Tendon disorders are highly debilitating and painful, and musculoskeletal injuries lead to more days off work than any other illness, costing the economy over £7 billion a year (1). Tendon injury is also common in horses (2) and with a total economic impact of over £3 billion a year from horse racing (3), preventing tendon injuries is high priority (4). A key outcome from our grant is improved understanding of tendon fatigue resistance which is fundamental to preventing damage. We anticipate considerable long term societal benefits to patients with tendon injuries, both preventing new cases and improving healing. This will improve quality of life, reduce strain on the NHS, and lower costs associated with time off work. Such findings are also directly relevant to horses. Around 16,000 race horses are in training each year (3), and tendon injury rate is as high as 43% with very few horses returning to racing post injury; preventing these injuries is essential for the economic health of the industry and to improve equine welfare (5). To realise these potential impact opportunities, we will target our research towards clinical colleagues and medical or healthcare companies.

This project will benefit academics and clinicians with an interest in tendon (dys)function. In the short term, our understanding of how tendons respond to load may clarify optimal methods for treating tendon injury. There is no current consensus on tendon treatment, and no clear physiotherapy regime to promote healing. The PI works closely with a clinical physiotherapist specialising in tendon disorders, and the pair have developed techniques for investigating in vivo tendon biomechanics. We anticipate using these systems to translate our in vitro findings to an in vivo setting and determine optimal physiotherapy training mechanisms for tendon repair. We also have a veterinary surgeon and academic within the current investigative team, enabling us to translate our research across veterinary boundaries. Discussions with these healthcare partners will not only provide valuable feedback on the clinical relevance of our data, but also allow us to consider the optimal methods of disseminating our findings in a manner accessible to healthcare professionals.

Towards the end of the grant we will focus on R&D investment. In characterising the key matrix components that protect tendon from damage, this work will be of interest to companies keen to develop products to prevent or treat tendon injury. There is also strong potential for collaboration with biomaterials and tissue engineering companies. The composite structure of tendon appears key to its optimal function and fatigue resistance. Characterising this will enable us to identify the specific material requirements that must be recapitulated in artificial tendons, significantly improving our potential for developing functional repairs. With skills in biomaterials amongst the applicants, we would remain closely involved in the development of repair solutions. The team has experience of working with medical device companies focused on tissue implant products (e.g. Tissue Science Laboratories; now Coviden), enabling us to advance research towards biomaterial tendon repairs. We also predict tissue engineering benefits, specifically relating to the biological and mechanical environment surrounding cells in the non-collagenous tendon matrix. We anticipate that our data will highlight how cell environment differs in healthy and damaged tendon, providing insights into the optimal in vitro environment for promoting tendon repair. As an additional area of expertise within the research team, we would aim to develop links with tissue engineering companies to take this forward.
1 Bevan 2007 Pub The Work Foundation
2 Clegg 2012 Equine Vet J 44:371
3 Deloitte LLP 2009 Pub British Horseracing Authority
4 HBLB 2012 Scope of veterinary research interests & current specific priorities
5 Dowling 2000 Equine Vet J 32:369